Transfer cases used on vehicles equipped with part-time four-wheel drive generally include transfer gear shift means for shifting a transfer gear assembly between a high and a low speed drive range, and center-differential gear shift means for shifting center-differential gear assembly between a differential-locked and a differential-free drive mode of operation and/or between a four-wheel and a two-wheel drive mode of operation. The shift means needs to be remotely operated within the driver's compartment to selectively shift the center-differential gear assembly to a desired drive mode of operation. For easy and positive remote operation, the center-differential gear shift mean is operated by means of an electric actuator.
One such transfer case with an electric actuator operating the center-differential gear shift means is disclosed in, for example, Japanese Patent Application No. 57-191,166 entitled "Four-wheel Drive Vehicle" filed on Oct. 29, 1982 and laid open as Japanese Patent Unexamined Publication No. 59-81,223 on May 10, 1984.
In the case of the transfer case taught by the above mentioned application provided with means such as free-wheeling means for connecting and disconnecting the transmission of power between the power train and selectively driven wheels, it is unavoidably necessary to couple a power transmission system for the selectively driven wheels to a power train in synchronism with a power transmission system for the regularly driven wheels upon shifting the transfer case into a four-wheel from a two-wheel drive mode of operation. For such a synchronized coupling, the transfer gear shift means is necessarily operated with a strong driving force.
Contrary to this, when shifting the center-differential gear assembly back to the two-wheel drive mode of operation, the differential shift means is operable lightly. For example, a transfer case used in cooperation with the free-wheeling means needs a driving force of, for example, approximately 15 to 20 kg for shifting the center-differential gear assembly into the twowheel from the four-wheel drive mode of operation and of, for example, approximately 80 to 100 kg into the four-wheel from the two-wheel drive mode of operation.
For this reason, the actuator for the differential gear shift means is generally designed to provide a strong driving force acting on a slidable clutch element enough for shifting the center-differential into the four-wheel from the two-wheel drive mode of operation.
One serious problem in association with the transfer case in which a center-differential gear assembly is shifted between the two-wheel and the four-wheel drive mode of operation by means of an electric actuator is an operational shock and noises that occur upon forcing to shift the transfer case, in particular the center-differential gear assembly of the transfer case into the two-wheel and the four-wheel drive mode of operation. This results from the fact that a torsional stress is produced in an interconnection between the driving systems for the front and rear wheels due to the difference of rotational speed.